This is a revised application for the competitive renewal of R37 AI36082 "Neutralization of primate immunodeficiency viruses", a grant first awarded in 1994, successfully re-competed in 1998, converted into an R37 ("MERIT") award at that time, and now up for re-competition once more. For most of the past 13 years, the emphasis of the research conducted with the support of this grant has been on learning how HIV-1 is neutralized by antibodies (Abs), or resists them, in the hope that such knowledge could facilitate the development of a vaccine based on the induction of neutralizing antibodies (NAbs). Such studies will continue to be an emphasis of the new application, particularly in Specific Aims 1 and 2. However, we now wish to initiate a new area of research, one that still involves learning how the HIV-1 envelope glycoproteins interact with the human immune system, but with no specific focus on neutralizing antibodies per se. Thus, in Specific Aim 3, we seek to characterize the potentially immunosuppressive signals that are transduced when the gp120 glycoprotein, at concentrations relevant to subunit vaccination, binds to human dendritic cells. We propose: Specific Aim 1: Mechanism, stoichiometry and kinetic aspects of HIV-1 neutralization by Abs. We seek to generate neutralization data that we will analyze using novel mathematical models, with the goal of better understanding the mechanisms of neutralization at the molecular level. Specific Aim 2: Can neutralizing antibodies synergize in their actions against HIV-1? Neutralization models suggest that the binding of a single Ab to an Env complex is sufficient to impair the fusion activity of that particular complex. Yet synergy has been reported between two or more NAbs, implying that multiple NAbs can bind the same trimer. We plan to analyze this apparent paradox experimentally, studying plausible sources of artifacts that could generate misleading indications of synergy. Specific Aim 3: Induction of immunosuppressive responses by the mannose moieties of gp120 glycans. We propose to examine what consequences vaccine-relevant gp120 concentrations can have on cells of the immune system, particularly dendritic cells. Our emphasis will be on determining whether the binding of gp120 to cell surface receptors, particularly mannose C-type lectin receptors, can generate immunosuppressive or cytokine-polarizing signals that skew the development of immune responses to this, and conceivably other, vaccine immunogens. PUBLIC HEALTH RELEVANCE One of the ways that HIV-1 is fought by the human immune system is via neutralizing antibodies that bind to the viral envelope glycoproteins and prevent it from infecting immune system cells that bear the relevant receptors. Most current vaccine design strategies involve the use of proteins intended to generate neutralizing antibodies, but these strategies are not working well because HIV-1 has evolved multiple mechanisms to resist the actions of these antibodies. In this grant application, we seek to generate new knowledge of how HIV-1 is neutralized by, and resists, antibodies, in the hope that the information could help with the design of better vaccine strategies. We also propose to study how HIV-1 envelope glycoproteins bind directly to some immune system cells and impair their functions, again with the intent of finding ways to make better vaccines that do not have these potentially suppressive effects.